Flash memory is used in a wide variety of electronic applications. Some flash memory cells utilize a floating gate field-effect transistor (FET), which stores one or more bits of data in the form of an electric charge within a “floating” gate. The floating gate resides between a channel region and a control gate of the FET, but is electrically-isolated from both by an oxide layer. Data is written to the memory cell when the FET is in an “on” state (i.e., when current flows between a source and drain) by applying a voltage to the control gate, which causes electrons to tunnel from the channel region into the floating gate. Because the floating gate is electrically-isolated from the channel region and the control gate, electrons that tunnel into it will remain there indefinitely.
Electric charge trapped within the floating gate screens the electric field from the control gate within the channel region, which selectively changes the threshold voltage (Vt) of the FET. For flash memory devices that use an array of memory cells, the stored data can be read out of the array by measuring which cells have a higher Vt (e.g., store a “1”) and which cells have a lower Vt (e.g., store a “0”). Multi-bit cells are also possible, where a single memory cell has more than two discrete Vt states corresponding to more than two data states.